block/rbd: add iotest for rbd legacy keyvalue filename parsing

block/rbd: add iotest for rbd legacy keyvalue filename parsing

This is a small test that will check for the ability to parse
both legacy and modern options for rbd.

The way the test is set up is for failure to occur, but without
having to wait to timeout on a non-existent rbd server.  The error
messages in the success path show that the arguments were parsed.

The failure behavior prior to the patch series that has this test, is
qemu-img complaining about mandatory options (e.g. 'pool') not being
provided.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Message-id: f830580e339b974a83ed4870d11adcdc17f49a47.1536704901.git.jcody@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>
(cherry picked from commit 66e6a735e97450ac50fcaf40f78600c688534cae)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>